Polymer electrolyte fuel cells generate electricity and heat simultaneously by allowing fuel gas (reaction gas) such as hydrogen and oxidant gas (reaction gas) such as air to electrochemically react with each other on a gas diffusion electrode having a catalyst layer containing an electrode catalyst (e.g., platinum) with the use of a hydrogen ion conductive polymer electrolyte membrane that selectively transports cations (hydrogen ions). FIG. 8 shows the typical structure of a polymer electrolyte fuel cell.
In a polymer electrolyte fuel cell 200, to both surfaces of a polymer electrolyte membrane 101 are attached catalyst layers 102A and 102B each composed mainly of a carbon powder carrying an electrode catalyst (e.g., platinum metal). To the outer surfaces of the catalyst layers 102A and 102B are attached a pair of gas diffusion layers 111A and 111B each comprising a fibrous substrate 104A, 104B and a water-repellent carbon layer (covering layer) 103A, 103B. The water-repellent carbon layers 103A and 103B have current collecting capability, gas permeability and water repellency. The catalyst layer 102A, 102B and the gas diffusion layer 111A, 111B are combined respectively to form gas diffusion electrodes. The polymer electrolyte membrane 101, the catalyst layers 102A and 102B, and the gas diffusion layers 111A and 111B are combined to form a membrane electrode assembly (MEA) 105.
To mechanically fix the membrane electrode assembly 105 and to connect adjacent MEAs 105 in series to each other, conductive separator plates 106A and 106B are inserted between MEAs 105. The separator plates 106A and 106B each have a gas flow channel 107A, 107B for supplying fuel gas or oxidant gas to the catalyst layer of fuel electrode or oxidant electrode on one surface thereof and a cooling water flow channel 108 for cooling the MEA 105 on the other surface thereof. Further, sealants 109 for preventing the reaction gas from leaking out are arranged.
The MEA 105 and a pair of separator plates 106A and 106B are combined to form a unit cell. A plurality of unit cells are stacked to form a cell stack. The cell stack is clamped in the stacking direction with clamping bolts 110 at a set clamping pressure so as to prevent fuel gas or oxidant gas from leaking or to reduce contact resistance in the stack. Accordingly, the MEA 105 is in surface contact with the separator plates 106A and 106B at a predetermined pressure.
In order to effectively utilize the reaction area of catalyst layer and to yield high cell output, or to improve assembly efficiency during the assembly of MEAs, usually, a structure as shown in FIG. 8 is employed. The main surfaces of the gas diffusion layers 111A and 111B is one size larger than those of the catalyst layers 102A and 102B. In the center portion of the gas diffusion layer 111A, 111B is placed the catalyst layer 102A, 102B. Accordingly, the perimeter (peripheral portion) of the gas diffusion layer 111A, 111B is positioned outside the main surface of the catalyst layer 102A, 102B (see, e.g., Patent Document 1).
However, the above conventional technique is accompanied by a problem: the peripheral portion of the gas diffusion layer 111A, 111B comes in direct contact with the polymer electrolyte membrane 101 at both fuel electrode side and air electrode side, so that the asperity on the surface of the peripheral portion of the gas diffusion layer 111A, 111B is likely to cause damage to the polymer electrolyte membrane 101 particularly during long-term operation. The surface asperity of the peripheral portion of the gas diffusion layer 111A, 111B is due to the structure and shape of the fibrous substrate 104A, 104B, and the above-described problem occurs regardless of the presence or absence of the water repellent carbon layer 103A, 103B.
Further, a cell stack of a conventional polymer electrolyte fuel cell is often clamped in the stacking direction at four points with clamping bolts 110, nuts (not shown) and clamping plates such that pressure is applied uniformly to the surface of the membrane electrode assembly 105 so as to reduce contact resistance. It is difficult to apply pressure uniformly throughout the surface of the membrane electrode assembly by the four-point clamping, however. The pressure applied to the portions where the clamping bolts 110 are positioned, i.e., the pressure applied to the peripheral portion of the gas diffusion layer 111A, 111B, is higher than the pressure applied to the center portion of the gas diffusion layer 111A, 111B. As such, a tendency is apparent, at both fuel electrode side and air electrode side, in which the peripheral portion of the gas diffusion layer 111A, 111B contacts the polymer electrolyte membrane 101 directly and more firmly, and the polymer electrolyte membrane 101 is likely to suffer damage.
If the polymer electrolyte membrane 101 is damaged, causing a through hole large enough to allow the reaction gas to leak, oxidant gas may be introduced into fuel gas, or fuel gas may be introduced into oxidant gas. If the mixed gas caused by the leakage is reacted by the action of the electrode catalyst, its reaction heat will cause further damage to the polymer electrolyte membrane 101. This may cause a decrease in output voltage or a termination of operation. Even if the damage is not so severe as to cause reaction gas leakage, there is a possibility that an electrical short-circuit might be caused between the fuel electrode and the air electrode. In this case also, the problem of a decrease in output voltage occurs.
As another approach, Patent Documents 2 and 3, for example, propose methods for solving the above problems in which a thin resin film having a thickness of several ten μm is formed on the peripheral portion of the catalyst layers so as to protect the polymer electrolyte membrane from the gas diffusion layers.
Patent Document 4 proposes a technique for protecting the peripheral portion of the polymer electrolyte membrane from damage by increasing the thickness of the peripheral portion relative to that of the center portion, in other words, the portion to be in contact with the catalyst layer (electrode reaction area) while maintaining the proton conductivity in the electrode reaction area.
Furthermore, Patent Document 5 proposes a technique to produce a polymer electrolyte membrane comprising a center portion made of hydrogen ion conductive material and a peripheral portion made of non-hydrogen ion conductive material resistant to shearing stress and heat.    Patent Document 1: JP 2002-208413 A    Patent Document 2: JP 5-174845 A    Patent Document 3: JP 8-185872 A    Patent Document 4: JP 8-185881 A    Patent Document 5: JP 2000-215903 A